A riser pipe is used to connect a subsea wellhead structure at the sea floor with a floating platform maintained in position above the wellhead. The riser pipe is subjected to an infinite number of degrees of freedom of movement. Both upper and lower ends of the riser pipe are affected by movement of a floating platform. Load parameters acting on a riser pipe system include forces resulting from heave, pitch, roll, sway or surge, height and period of swell, specific gravity of fluid conducted within the riser, buoyancy means attached to the riser, current profile, offset of the platform from the axis of the well means, tension at the top of the riser, and the stiffness of the riser relative to rotation thereof and its vertical disposition.
The term "fixed" or "fixed connection" is used to identify a non-rotatable or non-bendable condition of a riser end portion about an axis perpendicular to the longitudinal axis of the pipe within a hawse pipe or well casing. The term "non-fixed" as used herein refers to a rotatable riser condition, that is, riser end portion are bendable at and within a hawse pipe or well casing.
Generally speaking, prior conventional riser constructions have provided a fixed connection to a wellhead at the seabed formation; the fixed connection initially being rigid and lately being in the form of a swivel means such as a ball joint or its equivalent so that when lateral bending forces acted on the riser pipe, the lower end of the riser pipe could pivotally adjust to displacement or movement of the pipe. The upper end of such prior risers were connected to the platform means by relatively complex riser tensioning systems which permitted the platform to move relative to the riser and at the same time maintain a desired tension on the riser. Thus, in some prior risers, the upper end of the riser and the platform means had relative movement longitudinally of the riser and often had a sliding relationship with the platform. In some further prior riser systems, the connection of the top end of the riser to the platform included a swivel arrangement which permitted the pipe to adjust relative to the platform as the platform became laterally displaced. In all such prior proposed risers with fixed end connections known to me, relatively severe bending stresses would be imparted to the riser pipe just above the wellhead structure and just below the platform. In addition, lateral movement of the platform means with respect to the wellhead caused a relative change in distance between the vertical position of the platform with respect to a fixed point on the seabed as evidenced by the angular rotation or displacement of the tethering lines and the change in vertical position of the top end of the riser. Any lateral displacement of the platform produces a change in tension and length of the anchor legs and of a riser pipe having at least one of its ends fixed.
In riser systems designed for deep water, it is desirable that the relative change in length between a riser extending from a well hole to a floating platform thereabove be minimized and that a construction and arrangement be used which would reduce bending stresses of the riser pipe adjacent to the seabed and adjacent to the platform as much as possible. Prior riser systems, with a fixed wellhead connection and a riser tensioned platform connection, would have been subjected to severe bending stresses at the above locations in the event of strong wind and wave currents.